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Surface forces between highly charged cationic polyelectrolytes adsorbed to silica:How control of pH and the adsorbed amount determines the net surface charge

Huang, Junhao; Liu, Xiaoyan; Thormann, Esben

Published in:Langmuir

Link to article, DOI:10.1021/acs.langmuir.8b00909

Publication date:2018

Document VersionPeer reviewed version

Link back to DTU Orbit

Citation (APA):Huang, J., Liu, X., & Thormann, E. (2018). Surface forces between highly charged cationic polyelectrolytesadsorbed to silica: How control of pH and the adsorbed amount determines the net surface charge. Langmuir,34(25), 7264-7271. https://doi.org/10.1021/acs.langmuir.8b00909

1

Surface forces between highly charged cationic

polyelectrolytes adsorbed to silica: how control

of pH and the adsorbed amount determines the

net surface charge

Junhao Huang1, Xiaoyan Liu1,2 and Esben Thormann*,1

1 Department of Chemistry, Technical University of Denmark, 2800 Kgs. Lyngby,

Denmark

2 Centre for Oil and Gas, Technical University of Denmark, 2800 Kgs. Lyngby,

Denmark

KEYWORDS: poly(diallyl dimethyl ammonium chloride), charge reversal, pH dependent,

adsorption conformation, atomic force microscopy, quartz crystal microbalance with

dissipation

2

ABSTRACT. Atomic force microscopy (AFM) and quartz crystal microbalance with

dissipation (QCM-D) were employed to investigate the pH dependent adsorption of

poly(diallyl dimethyl ammonium chloride) (polyDADMAC) to silica surfaces as well as the

surface forces between these layers. It was found that polyDADMAC adopted a relatively flat

conformation when adsorbed to a silica surface, and that the adsorbed amount increased with

increasing pH. From the surface force measurements it is evident that the surface undergoes a

charge reversal upon saturation with polyDADMAC, at the three different investigated pH

values and that some degree of charge regulation of the silica surface takes place during the

adsorption process. Finally, the overcharging phenomenon is discussed in terms of a

geometrical mismatch due to the different average spacing between the surface charges on the

silica surface and the size of the polyDADMAC monomer.

1. INTRODUCTION

Polyelectrolytes are charged polymers, which bear electrolyte group attached to polymer

chains and have strong interactions with oppositely charged surfaces in aqueous media1, 2, 3, 4.

Electrostatic attraction between charged surface groups is the main driving force for adsorption

of polyelectrolytes to surfaces with an opposite net charge but various factors such as the ionic

strength, the adsorbed amount of the polyelectrolyte, the charge densities of the polyelectrolyte

and the surface as well as charge regulation effects influence the adsorbed amount5. For low

ionic strength conditions, the strong repulsion between charged monomers makes highly

charged polyelectrolytes assume extended conformation and fairly inflexible conformation at

the interface, while at an increased salt concentration, the electrostatic intra-chain repulsions

will decrease and the polyelectrolyte can adopt a more loosely bound conformation6, 7. For

surfaces or polyelectrolytes with acidic or basic groups the pH is further of particular

importance for the adsorption process.

Poly(diallyl dimethyl ammonium chloride) (polyDADMAC) is a strong cationic polymer with

permanent positively charged quaternary ammonium group in each monomeric unit and it can

thus adsorb to negatively charged surfaces like silica (see Figure 1). This polyelectrolyte is

applied in many industrial fields, such as paper manufacturing8, wastewater treatment9 and

mining industry10 and the adsorption of polyDADMAC on solid surfaces has been studied

using different methods. Cakara and coworkers11 studied the charging behaviour of aqueous

silica particle suspensions in the presence of polyDADMAC by potentiometric titrations and

3

electrophoretic mobility, and found that those particles experienced a charge reversal with

increasing pH. Kim et al12 employed sum frequency spectroscopy to investigate the molecular

structural details of the adsorption of polyDADMAC at a quartz/water interface. Here it was

found that polyDADMAC was not well aligned at the interface at below pH 9 but became well-

ordered at higher pH-values. The kinetics of adsorption and monolayer of polyDADMAC were

investigated by Michna et al., using quartz crystal microbalance (QCM) and streaming

potential measurements13. They successfully interpreted the 3-dimensional electrokinetic

model and quantitatively analysed their desorption kinetics. Popa et al14 examined the

adsorption of polyDADMAC on planar silica substrates as a function of ionic strength and pH

using reflectometry, atomic force microscopy (AFM) and ellipsometry. They found the

adsorbed amount to increase with increasing ionic strength and pH. However, most of the

research focuses on the adsorption kinetics of polyDADMAC on planar substrates or on

particles, while the studies about the conformation on the surfaces are few.

In this work, we have studied the adsorption of polyDADMAC to silica surfaces using QCM-

D and the interactions between silica surfaces after the adsorption of polyDADMAC using

AFM colloidal probe, at different pH values and a background electrolyte solution containing

1 mM NaCl. The work has been focusing on the effect of overcharging which is observed when

surface saturation of polyDADMAC is reached at a specific pH value; the charge reversal

which is achieved when polyDADMAC is adsorbed at a lower pH value and the pH

subsequently is increased; and the different nature of the surfaces forces between saturated and

unsaturated polyDADMAC layers. Finally, the obtained data are used in a discussion about the

general mechanisms for overcharging during polyelectrolyte adsorption to an oppositely

charged surface.

Figure 1. The molecule structure of polyDADMAC and its behavioural regime on an oppositely charged

silica substrate

4

2. MATERIALS AND METHODS

2.1. Chemicals. The cationic polyelectrolyte poly(diallyl dimethyl ammonium chloride)

(polyDADMAC) was purchased from Polysciences, Inc. as an aqueous solution of a

concentration of 20%. The supplier reports the weight-average molecular weight is 240 kDa

and a polyDADMAC with a similar molecular weight from the same supplier has been reported

to have a polydispersity of 2.015. Sodium chloride (NaCl, 99.5%) was purchased from Sigma-

Aldrich. The ultrapure water was purified by using a Milli-Q plus 185 system with a 0.2 µm

Millipak filter at 25 °C. The resistivity of the purified water was 18.2 MΩ cm, and the organic

contents were less than 5 ppb. PolyDADMAC solutions of 200 ppm were prepared by diluting

the 20% polyDADMAC stock solutions in 1 mM NaCl aqueous solution. The pH of the

solutions was measured with an 827 pH meter (Metrohm, Swiss) and adjusted by the addition

of an appropriate amount of NaOH or HCl solutions to obtain 3, 5.6 and 9.

2.2. Quartz crystal microbalance with dissipation (QCM-D). The adsorption of

polyDADMAC on silica as a function of pH was investigated employing a Q-Sense E1 quartz

crystal microbalance with dissipation (QCM-D) equipped with a standard Q-Sense flow

module (Biolin Scientific, Gothenburg, Sweden) with a volume of 40 μL. All measurements

were performed on AT-cut quartz crystals with a SiO2 top coating and a 5 MHz fundamental

frequency from Q-Sense (QSX 303, Biolin Scientific, Gothenburg, Sweden). If the adsorbed

mass is evenly distributed, rigid and small compared to the mass of the crystal, the frequency

change Δf is related to the adsorbed mass per unit surface according to the Sauerbrey equation16:

𝛥𝑚 = −𝐶∆𝑓

𝑛 (1),

Where Δm is the mass change, C is the mass sensitivity constant depending on the physical

property of sensor, that is 0.177 mg·m-2·Hz-1 for the crystals used, and n is the overtone number

(in the present case n=3). For a polymer film in aqueous solution it should further be noted that

the mass obtained from a QCM measurement is the so-called wet mass which includes the

adsorbed polymer chains and the solvent molecules coupled to the oscillation.

Before using the silica crystal, it was repeatedly washed using acetone and copious amount

of Milli-Q water, dried with compressed air, and ultimately plasma-cleaned for 1 min (PDC-

32G plasma cleaner, Harrick Plasma). After mounting the quartz crystal in the flow module of

the apparatus, a degassed electrolyte solution (pH adjusted) was injected at a steady flow of

100 µL/min at 25 °C. After establishing a stable baseline for the frequency and dissipation, the

5

adsorption measurements were started by shifting from the electrolyte solution to a 200 ppm

polyDADMAC solution with the same salt concentration and pH. After 40 min, the cell was

rinsed with the pH adjusted salt solution in order to remove any residual polyelectrolyte. These

experiments were conducted in duplicates and the largest difference in frequency shift between

two replicates was 15 % which is a fair reproducibility considering the relatively low absolute

values of the frequency shifts.

2.3. Atomic force microscope (AFM). Surface force measurements employing the colloidal

probe technique17, 18 were conducted using a NanoWizard 3 atomic force microscope (JPK

Instruments AG, Berlin, Germany). Thermally oxidised silicon wafers with a 100 nm thick

SiO2 layer (Wafer Net, USA) were employed as the flat substrate for the force measurements

while a silica particle with a diameter of approximately 7 µm (Bangs Laboratoried Inc, USA)

was used as the colloidal probe. The size of the particle was determined using a Nikon Eclipse

LV100ND optical microscope and the Infinity Analyze image processing software. A tipless

rectangular cantilever (CSC38/Cr-Au, Mikromasch, Estonia) was employed for AFM force

measurement and a small amount of a two-component epoxy adhesive (Araldite, Rapid) was

used to glue the particle on the end of the cantilever. The value of the spring constant was

determined by the Sader method, before particle attachment19, 20. The oxidized silicon substrate

and all tools were repeatedly rinsed with acetone and copious amount of Milli-Q water before

being dried with compressed air. The cantilever with the silica particle and the silica substrate

were plasma-cleaned immediately before the experiments were conducted.

The normal forces between the bare silica surface and the bare silica probe in 1 mM NaCl

were firstly measured at pH 3, 5.6 and 9. Then a 200 ppm polyDADMAC solution in 1 mM

NaCl at pH 3 was injected and the polyelectrolyte was allowed to adsorb for 40 min. After

rinsing with the same background electrolyte solution, the normal forces were measured at

room temperature. The normal forces were repeated after increasing the pH using the 1 mM

NaCl solution at pH 5.6. Hereafter, the polyDADMAC solution in 1 mM NaCl at pH 5.6 was

introduced for 40 min before the 1 mM NaCl at pH 5.6 without polyDADMAC was injected.

The above processes were repeated after increasing the pH to 9 and adsorbing at pH 9. After

each adsorption step and subsequent rinsing as well as after increasing the pH, the system was

subjected to 15 min of stabilisation time before the force measurements were conducted. For

each measurement condition, more than 90 force curves were collected at various surface

positions, where an area of 10 µm × 10 µm was adopted equally. As will be discussed further

in the results section, these force curves show almost perfect reproducibility with respect to the

6

long-ranged electrostatic double layer repulsion measured during approach, but some spread

in the adhesion measured during retraction. A constant approach and retraction speed of 400

nm/s were adopted for the normal forces, and it is sufficiently slow to allow us to neglect

hydrodynamic effects21. The raw data and analysis of the force curves were processed and fitted

using the standard software of the instrument (JPK SPM Data Processing), as described in

detail elsewhere22, 23.

Theoretical force curves originating from the electrostatic double layer interaction were

obtained by solving the non-linear Poisson-Boltzmann equation using constant charge and

constant potential boundary conditions, respectively24, 25. For a symmetric background salt,

such as NaCl used in this study, the non-linear Poisson-Boltzmann equation is given as

𝑑2𝜓

𝑑𝑥2=

2𝑧𝑒𝜌∞

𝜀𝜀0sinh (

𝑧𝑒𝜓

𝑘𝑇) (2),

where ψ is the double layer potential, x is the position of a plane of equal potential away from

a flat surface, e is the elementary charge, ρ∞ is the background salt concentration (number

density), ε is the dielectric constant of the solution between the interacting surfaces, ε0 is the

permittivity of vacuum, and z is the ion valance (which is 1 in this case). For a more detailed

discussion on how to derive the theoretical force curves from the non-linear Poisson-

Boltzmann equation we refer to the previous work26. However, from the non-linear Poisson-

Boltzmann equation, the double layer pressure between two flat surfaces, ΔP, is obtained, and

can subsequently be converted to the free energy of interaction per unit area, ΔG, by:

∆𝐺(𝐷) = − ∫ ∆𝑃(𝐷)′𝑑𝐷′ (3).𝐷

∞

Finally, the free energy of interaction can be related to the force between a sphere with radius

R and a flat surface by the Derjaguin approximation27:

𝐹(𝐷) = 2𝜋𝑅∆𝐺(𝐷) (4).

In addition to the electrostatic double layer force, the experimental force curves will be

influenced by van der Waals forces, hydration forces and steric forces. Hydration and steric

forces will only kick-in at small surface separation and due to surface roughness effects we

also only expect the contribution from the van der Waals forces to be short-ranged28. Thus, our

approach for comparing experimental force curves with the calculated double interactions, in

order to determine the surface potentials, are as follows: First, the Debye length is kept fixed

7

(based on the ionic strength of the solution) leaving the surface potentials as the only adjustable

parameters. Second, at large separations the forces calculated by the two set of boundary

conditions overlays, and the surface potentials are adjusted to fit the experimental data in this

region. However, at short separations the force calculated using constant charge boundary

conditions is overestimating the real electrostatic double layer force while the force calculated

using constant potential boundary conditions is underestimating the real electrostatic double

layer force. This means that the experimentally obtained force always is found between the two

calculated force profiles at short separations. Since none of the used boundary conditions is

able to describe the force at short separations, it also means that the approach is insensitive to

the before mentioned forces contributing to the experimental force profile at short separations.

3. RESULTS AND DISCUSSION

3.1. Adsorption of polyDADMAC layers. In order to obtain information about the amount

and conformation of adsorbed polyDADMAC on silica surfaces at different pH values, QCM-

D was employed. The change in frequency and dissipation as a function of time for adsorption

of polyDADMAC at different pH values is shown in Figure 2. At pH 3 and 9, the frequency

and dissipation values reach equilibrium after approximately 50 s, while a slow adsorption

process followed the rapid increase in adsorption of polyDADMAC at pH 5.6. After

approximately 40 minutes, the same background aqueous electrolyte solutions were used to

rinse and to remove the excess polyelectrolyte from the cell. During the rinsing step, almost no

decrease in the frequency was observed. At pH 3, the silica surface is carrying a relatively low

surface charge density and the surface is rapidly saturated by polyDADMAC. Similarly at pH

9, where the surface charge density is high, the saturation is also fast. At pH 5.6 the surface

charge density is also relatively high but here a slightly slower adsorption is observed. We

speculate that the slower adsorption at pH 5.6 is due to conformational changes of the adsorbed

polyDADMAC and increased charge regulation of the silica surface following the adsorption

process but the exact adsorption mechanism is at this stage unknown.

The data in Figure 2B, also reveal very small changes in dissipation during adsorption of

polyDADMAC layers, implying that the polyDADMAC molecules adopted a reasonably flat

conformation on the silica surface. In previous investigations, it was found that for such a

small dissipation shift (that is ΔDn/(-Δfn/n) «4×10-7Hz-1 for a 5 MHz crystal), the film can be

approximated as rigid, and the Sauerbrey equation can be used to calculate the areal mass

8

density of the film13, 29. In doing so we have determined the adsorbed amount of polyDADMAC

and associated water molecules to be 0.09, 0.16 and 0.22 mg/m2 at pH 3, 5.6, and 9, respectively.

We will later relay these numbers to the effective surface charge densities for the adsorbed

layers at the same pH values.

Figure 2. Change in frequency and dissipation as a function of time for adsorption of polyDADMAC on

silica at different pH-values. Black, red and blue line and points represent pH 3, 5.6 and 9, respectively.

3.2. Interactions between bare silica surfaces. In order to evaluate the measured interactions

between adsorbed polyDADMAC layers adsorbed to silica at different pH values, it is essential

to first investigate the surface forces between bare silica surfaces. Thus, before addition of

polyDADMAC to the AFM liquid cell, the force versus distance between the silica sphere and

9

substrate were measured. Figure 3 shows the approach force curves for the silica surfaces in 1

mM NaCl at pH 3, 5.6 and 9, respectively. From the DLVO calculations the surface potential

of bare silica at pH 3 is determined to +/-20 mV, and the numerical value of the surface

potential increases to +/-44 mV and +/-48 mV at pH 5.6 and pH 9, respectively. From the force

measurements and the DLVO calculations, the sign of the surface potentials are not implicit,

but it is well known that silica surfaces are negatively charged due to dissociation of some

silanol groups30, 31. The dissociation is also the reason why the surface potentials increase with

increasing pH and the results are in qualitative agreement with zeta-potential measurements

which indicates a significant shift in potential in the pH-range 3-6 and a weaker pH dependence

above pH 612, 32. In the DLVO calculations, a Debye length of 6.8 nm was used at pH 3 while

a Debye length of 9.6 nm, corresponding to the Debye length of the background 1 mM NaCl

solution, was used at pH 5.6 and 9. This difference is rationalized by the extra ions added in

for the pH adjustment ([H+] = 10-pH) which is increasing the total ionic strength at pH 3 to 2

mM.

Figure 3. Surface forces between a bare silica substrate and a bare silica colloidal probe in 1 mM NaCl as

a function of pH. Black open squares, red open circles and blue open triangles represent pH 3, 5.6 and 9,

respectively. The graph shows in each case 10 consecutive approach force curves for each pH-value and the

inset shows the approach curves plotted on a semi-log scale. The red and blue lines are fitted with DLVO

forces using constant charge and constant potential boundary conditions, respectively.

3.3. Electric double layer forces between polyDADMAC layers. Measurements of the

interaction between adsorbed polyDADMAC layers at different pH values followed a

procedure with a stepwise change in pH and adsorption/resorption of polyDADMAC to the

10

silica surfaces. First polyDADMAC was adsorbed at pH 3 and the surface forces were

measured after rinsing the system in order to remove nonadsorbed polymers. Hereafter the pH

was increased to 5.6 and the surface forces were measured. Then the solution containing

polyDADMAC at pH 5.6 was reintroduced in order to saturate the surface at this pH value and

the surface forces were again measured after rinsing. The same procedure was followed when

the pH was increased to 9. This means that we have measured the interaction between fully

saturated adsorbed layers at pH 3, 5.6 and 9, while we have also measured the surface forces

between unsaturated adsorbed layers at pH 5.6 and 9. All adsorption, rinsing and force

measurements were conducted at a 1 mM NaCl solution (but at different pH values). Figure 4

shows representative forces curves from the different steps following this experimental

procedure. The insets in Figure 4 provide the force curves on approach in a semi-log together

with calculated DLVO force curves.

11

Figure 4. Forces normalised by radius as a function of separation between silica surfaces coated with

PolyDADMAC in 1 mM NaCl solutions. Black open squares and red open circles represent data obtained

on approach and retraction, respectively. The graphs show in each case 10 consecutive approach and

retraction force curves and the insets show 10 consecutive approach force curves plotted on a semi-log scale.

The red and blue lines in the insets are the DLVO forces calculated using constant charge and constant

potential boundary conditions, respectively. The background electrolytes are 1 mM NaCl. (A) Force curves

obtained between bare silica surfaces at pH 3. (B) Force curves obtained at pH 3 for polyDADMAC

adsorbed at pH 3. (C) Force curves obtained at pH 5.6 for polyDADMAC adsorbed at pH 3. (D) Force

curves obtained at pH 5.6 for polyDADMAC adsorbed at pH 5.6. (E) Force curves obtained at pH 9 for

polyDADMAC adsorbed at pH 5.6. (D) Force curves obtained at pH 9 for polyDADMAC adsorbed at pH

12

9. The figures on the left panel are assigned to unsaturated adsorbed layers with net negative charges, while

the figures on the right panel belong to saturated adsorbed layers with net positive charges.

From Figure 4 it is observed that all the approach force curves show monotonically repulsive

interactions with a close to exponential decay, which is consistent with electrical double-layer

forces. Since the measured interaction lies between the values obtained using constant surface

charge and constant surface potential boundary conditions, we conclude that some charge

regulation occurs as the surface separation decreases. The surface potential obtained from the

DLVO calculation are presented in Tabel 1 where the arrows indicated the experimental route

and the letters in brackets refer to the subfigures in Figure 4 from where the numbers are

deduced.

Table 1. Surface potentials of silica at different conditions and pH. The arrows and letters (A to F) refer to

order in which the measurements were conducted and the corresponding results are shown in Figure 3A-

F.

pH 3 pH 5.6 pH 9

Surface potential of bare silica -20 mV (A) -44 mV -48 mV

Surface potential of silica saturated

with polyDADMAC

+36 mV (B) +53 mV (D) +52 mV (F)

Surface potential of silica

unsaturated with polyDADMAC

-38 mV (C) -39 mV (E)

Charge density of silica saturated

with polyDADMAC 1.77 × 1016 e·m-2 2.83 × 1016 e·m-2 2.76 × 1016 e·m-2

As discussed for the interaction between bare silica surfaces, electric double layer calculations

with respect to the interaction between symmetric surfaces do not distinguish between a

positive or a negative surface potential. Thus, additional information or rational arguments are

needed to determine the sign of the surface potential. Compared to the bare silica surfaces

where the surface charge could only originate from dissociated silanol groups (and thus only

be negative), the surface charges will now both originates from dissociated silanol groups and

from the permanent charges on polyDADMAC (and the total surface charge can thus be either

negative or positive). However, in the present case it is relatively easy to determine the sign

of the surface charge in each step. If the sign of the surface potential after polyDADMAC

adsorbed at pH 3 should be negative, the numerical value should be smaller than the value for

bare silica at pH 3 (because we are adding positive charges). Thus, since the numerical value

of the surface potential is higher after adsorption of polyDADMAC it can be concluded that

the surface potential must be positive. By similar arguments, it can also be concluded that the

13

signs of the surface potentials at pH 5.6 and 9 are negative for the surfaces with unsaturated

layers of polyDADMAC and positive for the surfaces with saturated layers of polyDADMAC.

For the saturated layers of polyDADMAC, this implies overcompensation of the surface charge

and hence charge reversal resulting in net positively charged surfaces which is consistent with

many previous reports26, 33. It is, however, interesting that the sign of the net charge of surface

layers reverses back to the negative value when the pH is increased and an unsaturated

polyDADMAC layer is obtained. When the pH increased from 3 to 5.6, the adsorbed amount

of polyDADMAC is fixed (and thus the amount of positive charges are fixed) but the silica

substrate turns more negative due dissociation of silanol groups resulting in an overall charge

reversal from positive to negative. When the pH is adjusted from 5.6 to 9, the conclusion about

the overall sign of the surface charge is less obvious since the measured net charge of +/-39

mV is below the numerical value of the surface potential measured for the saturated layer at

pH 5.6. However, we argue that the net charge of the unsaturated layer at pH 9 should be

negative since no significant further adsorption of polyDADMAC would be possible if the

surface was carrying a high net positive charge. It should further be noted that the observation

of a charge reversal depending on the degree of surface saturation with polyDADMAC at a

given pH value is in agreement with previously reported observation for adsorption of

polyDADMAC to different substrates. Michna et al13 used streaming potential measurement to

study the effect of surface coverage of polyDADMAC on mica at constant pH. Here it was

found that the streaming potential was changing from negative to positive as the coverage was

gradually increased. Schwarz et al34 measured the ζ-potential of silica, mica and acidic polymer

latex particles with different adsorbed amount of polyDADMAC. In this study, they found a

gradual increase in the isoelectric point of the particles as the adsorbed amount of

polyDADMAC on the particles was increased.

3.4. Polymer induces forces between adsorbed polyDADMAC layers. Beside the long-

ranged electrical double layer forces measured when the surfaces are approaching each other,

short-ranged polymer induced surface forces are in some cases observed in both the approach

and retraction force curves.

For the saturated layer of polyDADMAC at pH 3 (Figure 4B), the short-range interaction is

purely repulsive, and no hysteresis is observed between forces measured on approach and on

retraction. The lack of a steric repulsive barrier and no short-ranged compressive component

to the interaction profile implies a low amount of adsorbed polyelectrolytes which have adopted

14

a relatively flat conformation at the surfaces. Similar observations have previously been

reported for other polyelectrolytes adsorbed on mica and silica35, 36.

When the pH is increased from 3 to 5.6, more silanol groups dissociate and more available

binding sites for polyDADMAC becomes available on the silica surface. Thus, when two

unsaturated polyDADMAC layers are approaching at pH 5.6, at some separation distance,

polymers adsorbed on one of the surfaces will be able to extend and bind to available spots on

the opposite surface. In Figure 4C traces of such bridging interactions are seen at a separation

of approximately 5 nm. After the surfaces have been in contact more bridges will form which

is resulting in the dominating adhesion force seen in the retraction force curve. A similar

condition has been observed in experimental studies which polyethyleneimine was adsorbed to

silica37 and by model results on the interaction between surfaces containing adsorbed

polymers38. In Figure 5 a histogram of adhesion forces, based on more than 90 consecutive

force curves, for the unsaturated surface at pH 5.6 shows an average adhesion force of 0.88

mN/m. The presence of bridging forces upon approach suggests that the polyDADMAC

molecules are not fixed in an entirely immobile flat conformation but might show some

flexibility with an ability to extend slightly into the solutions. Such a slightly extended

conformation might also explain the significant overcharging seen from the long-ranged forces.

Figure 5. Histograms showing the distribution of adhesion forces (normalized by radius) between

unsaturated polyDADMAC coated silica surfaces at pH 5.6 (A) and pH 9 (B), respectively. These two

conditions are corresponding to the conditions in Figure 4C and Figure 4E and the adhesion forces

correspond to the maximum adhesion forces observed in the two sets of retraction force curves.

After reintroducing the polyDADMAC solution at pH 5.6, all the new available binding sites

will again be occupied, thus effectively preventing bridging interaction between these saturated

layers. In the approach curve in Figure 4D no “bump” appears but a small force minimum

observed upon retraction reveals that a minor degree of polymer bridging can take place after

15

the surfaces have been in hard contact, although, still to a much smaller extent as for the

unsaturated layers.

A very similar trend as seen when increasing the pH from 3 to 5.6 was found when increasing

the pH from 5.6 to 9. However, a significant difference is a stronger adhesion force observed

for the unsaturated layers at pH 9 (Figure 4E) compared to the unsaturated layers at pH 5.6

(Figure 4C). As seen from Figure 5 the adhesion force at pH 9 possesses an average value of

2.87 mN/m which is more than three times higher than the adhesion force found for the

unsaturated layers at pH 5.6.

3.5. The effect of overcharging. As evident from the AFM-based force measurement and the

derived values of surface potential and surface charge densities presented in Table 1, adsorption

of polyDADMAC is not only neutralizing the negative surface of silica but leading to a

significant overcharging of the surface. Noteworthy, almost an exact charge reversal is

observed at all three investigated pH value, however with a slightly higher numerical value of

the surface potentials and surface charge densities of the adsorbed polyDADMAC layers

compared to the bare silica surface.

By assuming that the adsorption of polyDADMAC first neutralizes the charges on silica and

next lead to an overcharging, one can in principle estimate the amount of adsorbed

polyDADMAC and compare the values with the sensed mass derived from the QCM-D

measurements. However, the sensed masses from the QCM-D measurements are at all three

pH values approximately one order of magnitude higher than the masses derived from the

surface charge densities. A similar discrepancy is described in a paper by Notley39 reporting

the adsorption of polyDADMAC in cellulose gels and several factors could be the reason for

this. Firstly, the assumption that the total amount of adsorbed polyDADMAC can simply be

determined by the summation of the surface charge densities of bare silica and an adsorbed

layer of polyDADMAC (multiplied by the mass of one DADMAC monomer), will likely

underestimate the adsorped amount due to charge regulation of the silica surface. This charge

regulation process which was already discussed in relation to the QCM-D data obtained at pH

5.6 (see Figure 2) means that more silanol groups dissociate as a response to the polyDADMAC

and that more positive charge thus are required for charge neutralization. Secondly, the

adsorbed amount for polyDADMAC determined from the surface potential measurements

might further by underestimated due to co-adsorption of chloride ions. According to the

classical theory such an ion condensation process, also often refer to as Manning

condensation40, 41, 42, will take place when the distance between two charges is smaller than the

16

Bjerrum length (lB=e2/(4πε0εkBT)). In the present case of the heavily charged strong

polyelectrolyte, polyDADMAC, the distance between two charges will compare with the

Bjerrum length, which is approximately 7 Å in water, and some minor degree of ion

condensation can thus be expected. Thirdly, it is well-known that the sensed mass obtained by

QCM-D is overestimating the real mass since it also includes water entrapped in the adsorbed

polymer layer39, 43, 44. Thus, taking these two factors into account will bring the values obtained

from surface force measurements and QCM-D closer to each other.

After arguing that the true amount of adsorbed polyDADMAC probably lies in between the

values derived from the AFM and QCM-D measurements, respectively, the next step will be

to discuss the physical reason for the large overcharging of the surface. Here, we believe that

the main reason is a mismatch of the charge densities on the silica and polyDADMAC which

is making a 1:1 charge neutralization impossible. E.g. at pH 9 where we have the highest charge

density of silica, the electrical double layer calculations returned a value of 2.48×1016 charges

per m2, which corresponds to an average distance of approximate 6-7 nm between two charges.

In comparison will the distance between two charges on the polyDADMAC chain correspond

to the size of one DADMAC monomer which is below 1 nm. Thus, for a polyDADMAC

molecule to neutralize neighboring charges on the silica surface it will have to bring several

extra positive charges due to this geometrical mismatch. On the regard, Kim and coworkers7

investigated the adsorption of polyDADMAC at the quartz/water interface employing IR-

visible sum frequency spectroscopy. They found that only at pH values higher than 9.6,

polyDADMAC chains were sufficiently well aligned at the interface to elicit a sum frequency.

Thus, at lower pH values it is reasonable to assume that segments of adsorbed polyDADMAC

layer extend slightly into the solutions even though our results generally suggest that

polyDADMAC molecules adopt a relatively flat conformation on silica surfaces. This is

possible also evidence from Figure 2B where the adsorbed polyDADMAC layer at pH 5.6 hold

bigger dissipation than that at pH 9 even though the sensed mass of adsorbed polyDADMAC

layer at pH 9 was bigger. By combining this knowledge, it is suggested that the pH influences

on not only the surface coverage but also the conformation of the adsorbed layer.

4. SUMMARY AND CONCLUSION

In this work, we first studied the adsorption of polyDADMAC on silica at different pH values

by QCM-D and secondly measured the surface forces between adsorbed layers as a function of

pH and adsorbed amount by colloidal probe AFM. The first part revealed a significant increase

in the adsorbed amount of polyDADMAC with increasing pH due to the dissociation of silanol

17

groups but also some pH dependence of the adsorption kinetics - especially at intermediate pH

values where the surface charge density is most pH sensitive. The surface force studies

demonstrated that the negative surface charge of bare silica was overcompensated for saturated

layers of polyDADMAC resulting in a surface charge reversal at all three investigated pH-

values. However, when polyDADMAC was first adsorbed at a lower pH-value whereafter the

pH was increased, the surface charge again changed to a negative value due to the increased

deprotonation of the silanol groups and the unsaturated nature of the polyDADMAC layer. The

difference between saturated and unsaturated polyDADMAC layers also manifested itself in

the short-ranged interaction and in the retraction force curves where attractive bridging

interactions are observed between the unsaturated polyDADMAC layers while purely repulsive

interactions were observed between the saturated layers. Finally, a discussion about the reason

for the large overcompensation of saturated polyDADMAC layers suggests that the charge

reversal and the adsorbed amount likely can be tuned by the degree of geometric mismatch

between the charge densities of the surface and the polyelectrolyte.

AUTHOR INFORMATION

To whom correspondence should be addressed. E-mail: esth@kemi.dtu.dk. Telephone: (+45)

4525 2439.

ACKNOWLEDGEMENTS

JH acknowledges a stipend from the Chinese Scholarship Council (CSC).

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